This discussion may contain spoilers!
After getting some dirt on Edna, I started to use the exact same method to trap the antagonist of the first part of the episode. This involves letting go of an item. It turns out you need this item in the post-expo part of the episode, if you left it behind you're outaluck and will need to do it all over again.
Explanation of the bug with spoilers:
After getting the recorded message of Edna and her running away from Detective Parker, it's possible to put the recorder back in the phone booth - I did so thinking it would help me trap elder Brown in soundproof and unbreakable glass while I fiddled with the Bathysphere controls. It turns out this is not intended behavior, you shouldn't be able to let go of the recorder after playing the confession. You can also check a clip on Youtube where you're able to talk to a phantom Edna because of it. Anyway, seeing my plan doesn't work out, I didn't bother to pick the recorder up, thinking it had done its job.
A while later you're trying to jog old Edna's memory about her almost-arrest by Parker. I spent half an hour trying to do a reenactment with the saloon sign on the outhouse and a burning stick, until I gave up and checked the clues. The first two told me stuff I already knew, but the third one was very strange, it told me to play back her confession. At this time I hadn't realized yet the hint was talking about the recorder, so I spent another 20 minutes using all the conversation options with Doc and talking to the cactus (I'm not crazy I swear :p) before I gave up and checked a walkthrough.
So yeah if anybody has a save at the start of the post-expo with the recorder in the inventory I'd appreciate it. As mentioned the fix is pretty simple, don't let Marty plant the plant (pun intended) after Edna runs away